Digital laminography—also called tomosynthesis—is a method of generating a three-dimensional representation of an object by means of X-raying that is not free from superimposed structures. It is particularly suitable for examining primarily flat objects. In digital laminography the object is irradiated from different directions. As a rule the irradiation takes place in discrete positions, which in linear laminography are in a plane that is (approximately) perpendicular to the principal plane of the object, and in rotational laminography are on a cone whose principal axis is (approximately) perpendicular to the principal plane of the test object. A complete data set for laminography is produced by a synchronous, opposed, generally linear or circular motion of radiation source and detector, relative to a reference point generally located in the object. Imaging is characterized by the motion of the detector relative to the reference point, the motion of the radiation source relative to the reference point, the tilt angle to the central ray, the number of projections, the detector-reference point distance and the radiation source-reference point distance. It is unimportant whether this relative motion is produced by the positioning of the imaging system (radiation source and detector) around the object at rest, or by the motion of individual or several components around the object, which is also moving, or only from the motion of the object while the imaging system is stationary.
A complete laminography data set for the test zone located around the reference point is produced by rotating the object by means of a rotating unit about an axis passing through the object. The imaging system, consisting of radiation source and detector, is tilted at a defined angle to the rotation axis. The central ray meets the object exactly or approximately where the rotation axis pierces the object. The reference point is defined as the point of intersection of rotation axis and central ray. Relative to this reference point, radiation source and detector virtually perform a synchronous, opposed circular motion.
A variant of the set-up just described envisages that the imaging system is oriented horizontally; the rotation axis is, accordingly, tilted at an angle to the central ray. Radiation source and detector perform a synchronous, opposed circular motion relative to the reference point. The aforementioned boundary conditions remain valid.
A disadvantage with these arrangements is that the size of the rotating unit limits the maximum size of the test object; in particular the production of precise positioning units for large and heavy objects is expensive and they must in each case be manufactured individually. Another disadvantage is that the mechanical realization of the rotational motion essentially defines the position of the reference point; in order to change the position of the test zone, the test object must be moved within the rotating unit relative to the reference point.
However, it is desirable to be able to choose the position of the reference point freely in the whole test object. To achieve this, either the object must be arranged movably within the rotating unit, i.e. another movement unit is necessary, which must be rotated in the system, or the rotating unit must be movable in the X/Z direction, i.e. perpendicular to the rotation axis, so that the position of the eccentrically arranged reference point is moved into the rotation axis by an X/Y motion.